This invention relates to optical disc drives, and particularly to high density optical disc drives that employ lenses supported on sliders that aerodynamically control head/disc spacing.
Run-out, in an optical disc drive, is the condition of variations in distance between the head and disc surface that leads to defocus of the objective lens. Run-out is caused by a lack of parallelism between the lens and disc surface, which in turn is caused by a number of factors, including unbalanced forces acting on the disc drive spindle, vibration, external noise, disc warpage, lack of perpendicularity of the disc spindle, and lateral motion of the spindle caused by ball bearing imperfections, to name a few. Many optical disc drives employ an objective lens positioned at a fixed location relative to the platform supporting the disc. Variations in the flatness in the disc, therefore, is the most common cause of run-out in such optical disc drives. Where the objective lens is mounted to a flying slider, disc run-out will be of less impact on lens focus because the slider follows undulations of the disc surface.
The density, or radial spacing, between concentric data tracks, on optical discs continue to increase, requiring greater precision of lens positioning. Conventional drives employing flying sliders accomplish lens positioning by operating an actuator arm with a large scale motor, such as a voice coil motor, to position a lens.backslash.slider on a gimbal at the end of the actuator arm. However, the large scale motor lacks sufficient resolution to effectively accommodate high track-density discs. Thus, a high resolution lens positioning mechanism is necessary to accommodate more densely spaced tracks.